Previous studies have shown that mother-reared rhesus monkey (Macaca mulatta) infants have higher cortisol levels than nursery-reared infants. Rodent studies have demonstrated the passage of maternally ingested radiolabelled glucocorticoids to offspring through infant milk ingestion. This suggests one possible mechanism by which mother-reared infants have higher cortisol concentrations than do nursery-reared infants. Here we present data from a larger assessment investigating mother-infant dyads that are socially housed at the California National Primate Research Center. Infants and mothers were relocated from their half-acre field cages and separated from each other. Blood was drawn from the infants two and eight hours after separation, and milk was collected from mothers approximately four hours after separation. Cortisol concentrations in blood and milk were assayed by radioimmunoassay. For these analyses, subjects were selected based on the infants’ cortisol response to separation and relocation; milk from mothers of the infants with the highest (n=8) and lowest (n=8) average cortisol concentrations was assayed. Cortisol concentrations in rhesus monkey milk are substantial (range: 2.77-27.44 ug/dl, mean=11.54 ug/dl, SD=7.72). Differences between infants in cortisol response to separation and relocation, however, were not related to concentrations of cortisol in maternal milk. It may be that maternal cortisol plays an important role in infant hypothalamic-pituitary-adrenal activity under basal conditions, but if so it is not reflected in the response to stress in the mother’s absence.